A non-porous substrate is any paper, metallized paper or plastic that will not absorb any ink oil that is present in a conventional ink. Because of this, special high solids inks are required. Additionally, non-porous substrates do not absorb water and thus the press tends to run wetter which affects the inks.
Inks manufactured for non-porous substrates contain 100% solid vehicles and large amounts of drying oils. These inks dry by oxidization instead of penetration and evaporation as do conventional printing inks. This means that oxygen must be present in sufficient amounts to allow the ink to dry. They also contain other ingredients to allow them to perform well on the press, transfer well to the substrate and still set fast enough to minimize the amount of anti-offset spray powder that is used. Oxidizing inks will still not set as fast as a conventional ink would on a porous substrate like gloss coated but the slow setting and fast drying insures that the inks will adhere to the stock.
When running non-porous substrates the biggest concern is ensuring the inks will dry. When running conventional inks and conventional paper the excess water can often be absorbed by the substrate. When running non-porous substrates the substrate cannot carry it away and so that fountain solution is absorbed by the ink. In order to make sure that the ink can still dry with this extra water some care must be taken.
Start with fresh fountain solution. As the fountain solution gets used certain chemicals leach out of the papers and inks to make them more readily absorbed by the ink. This is bad because the inks will already be carrying more water when running the non-porous substrate.
When printing on non-porous substrates, most ink manufacturers prefer the use of isopropyl alcohol for two reasons. The first is it makes the water wetter, which translates into being able to run less water and the second is they evaporate readily out of the ink. The alternative to isopropyl alcohol is an alcohol replacement. These chemicals make the water wetter but remain in the ink while it is drying. The most important thing from the ink drying point of view is to run the minimum amount of fountain solution to get the job to print.
Ink driers can be mixed into the ink which is then emulsified on press. This releases oxygen into the ink film. This can reduce drying time by as much as 50%. Another useful additive when printing non-porous substrates is an ink drying stimulator containing water soluble cobalt – the most active drier available. By adding it to the water, it can also help the ink dry faster. This is preferred to adding the cobalt to the ink because it does not affect the ink on the rollers and it increases ink concentration as the ink absorbs more water and needs more help drying.
Spray Powder is an often overlooked component of successful printing on plastic. The right size powder will give separation of the sheets. This affect is two-fold. First when the sheet initially delivers, it is less likely to slap together and cause ink transfer. Second, by giving separation of the sheets, more oxygen is able to flow into the pile and help dry that job. The size of powderrequired varies by print-job because non-porous substrates vary greatly in weight.
Further Points to Consider
There are many things that can affect any print job:
- Static eliminators help the sheets feed well and not slap together in the delivery.
- Stay-open sprays can entrap oxygen but without oxygen, oxidizing inks cannot dry.
- Low pH can affect ink drying but so can running a higher pH by not adding enough etch. Allow the minimum amount of fountain solution to be run.
- Aqueous coating is a common printing practice. On non-porous substrates they allow the printer to run high piles and possibly back up jobs more quickly. However, oxidizing inks require oxygen to dry and an aqueous coating seals that ink away from the air. This may prevent the job from being further processed in a timely fashion.